How does Carbohydrate Restriction affect Blood Sugar Levels?

Restricting carbohydrates can help patients with diabetes manage glycemic control.

As a chronic and progressive disease that can lead to adverse health outcomes, patients with diabetes must monitor biological markers such as blood glucose levels as well food intake. Glycemic control, understood as a HbA1c < 7 percent, is considered the foundation for managing diabetes¹. Poorly controlled blood glucose levels have been identified as causes for negative health outcomes such as cardiovascular disease, retinopathy, and neuropathy, to name only a few.  

A study from the U.K. found that when HbA1c decreased by one percent through monitoring one’s diet and adherence to pharmacological interventions, diabetes-related deaths were reduced by 21 percent.² While it is critically important for patients with diabetes to comply with their doctor’s instructions for taking medications, reliance on medications to control diabetes may be reduced by lifestyle changes, such as adopting a restricted carbohydrate diet. Consequently, it is important to understand the role of diet in diabetes. 

Carbohydrates, HbA1c, and Weight Change

A recent systematic review¹ looked at 25 studies that investigated the weighted mean difference for HbA1c between participants assigned to either a carbohydrate restricted or high carbohydrate group (N = 2,412). Participants in the reviewed studies include persons with obesity, ranging in age from 52–63. Seventy five percent of participants in the reviewed study had adequately controlled diabetes, with an HbA1c of < 8 percent while the balance of the participants had suboptimal AbA1c, ranging from eight percent to 9.1 percent. 

A meta-analysis¹ revealed that there was an overall greater reduction in HbA1c at three months for participants in the carbohydrate-restricted groups. This difference is attributed solely to the low carbohydrate diet. Subgroup analyses also revealed a greater reduction in HbA1c for the low carbohydrate groups. However, at 12 months and 24 months, there were no significant differences between the diet groups. The carbohydrate-restricted diet produced a .19 percent reduction over six months in HbA1c, which is a clinically significant reduction.

In addition to lowering HbA1c, greater weight loss was observed in the carbohydrate-restricted diets and subgroup analyses revealed this was attributed to the decrease carbohydrate intake, < 26 percent of total energy intake. However, at six months and 12 months, no significant differences in weight loss were observed. This review suggests that carbohydrate-restricted diets are efficacious in the short term (3 – 6 months). For optimal HbA1c reductions in 3–6 months, it is suggested that patients should limit their carbohydrates to 26 percent of their energy intake. The weight loss associated with carbohydrate-restricted diets is believed to contribute to the reduce HbA1c levels. More moderate reductions between 26–45 percent of total energy intake was not found to confer additional benefits. 

Carbohydrate-Restricted Diets

While there is still a need to further investigate the long-term impacts of a carbohydrate-restricted diet, there is strong evidence for their efficacy in the short term, specifically as it relates to glycemic control. For patients with obesity and diabetes, a supervised weight loss program that includes a short-term carbohydrate-restricted diet (3-6 months) as part of a Low Calorie Diet (LCD)may provide them the jumpstart they need to make lifestyle changes that can be sustained long-term and yield long-term results.  

Sources:

1. Effect of dietary carbohydrate restriction on glycemic control in adults with diabetes: a systematic review and meta-analysis
2. Association of glycaemia with macrovascular and microvascular complications of type 2 diabetes (UKPDS 35): prospective observational study

About the Author: Dr. Dawn  M. Sweet has over 20 years of experience in the field of communication. Dr. Sweet has given several invited talks to and workshops for academic and private sector audiences on the role of nonverbal and verbal communication in achieving positive outcomes and mitigating bias. Her research has been published in several top ranked peer-review journals, and it has been featured on NPR’s River to River / All Things Considered, Buzzfeed, and Science Daily. Her research has also been used to inform expert testimony.